Electroabsorption modulators are generally known. These devices can be fabricated in several compound semiconductor systems (e.g., AlGaAs), and typically include a heterojunction waveguide on a substrate. A modulated bias voltage is applied to the device through electrode contacts to reverse bias the pn junction and set up an electric field within the waveguide. The band gap energy of the waveguide semiconductor material is greater than the photon energy of the light to be modulated. Incident light therefore propagates through the device in the absence of the applied bias voltage. However, the electric field produced by the bias voltage causes an increase in the absorption coefficient in accordance with the Franz-Keldysh effect. Light propagating through the waveguide is therefore modulated by applying a modulated bias voltage to the device.
A number of competing factors must be optimized to produce practical electroabsorption modulators. Insertion losses, including both losses within the waveguide at zero applied bias (i.e., zero-bias absorption) and coupling losses, must be low. This factor limits how close the semiconductor band gap energy can be to the photon energy of the light to be modulated, since zero-bias absorption increases for photon energies near the edge.
The extinction ratio (i.e., the amount of light absorbed when bias voltage is applied) should be large. Franz-Keldysh absorption is greatest when the waveguide semiconductor band gap edge is just beyond the photon energy of the light to be absorbed. The band gap of the waveguide semiconductor must therefore be selected as a compromise for both low insertion loss and high extinction ratio.
Operating voltage should be minimized to limit the amount of heat dissipated by the device. The absorbed light generates photocurrent within the device. This photocurrent is multiplied by impact ionization caused by the electric field, further increasing the power which must be dissipated.
Known electroabsorption modulators typically have a relatively thick undoped waveguide region (1-2 .mu.m) across which the electric field is distributed. These devices require relatively high modulating bias voltages (50-100 V) to achieve the high electric fields (0.5 mv/cm) needed for strong electroabsorption. Since the absorbed light generates photocurrent, the efficiency of these devices is relatively low (2-4%). Decreasing the difference between the photon energy of the light and the band gap edge of the semiconductor (e.g., to a difference of less than 80 meV) to increase electroabsorption is not practical due to the increased zero-bias loss.
It is evident that there is a continuing need for improved electroabsorption modulators. A low power, low insertion loss and high extinction ratio modulator of this type for visible laser light would have widespread application. To be commercially viable, the device must also be efficient to manufacture.